Electric discharge tube for short waves



G. DIEMER ET AL ELECTRIC DISCHARGE TUBE FOR SHORT WAVES Filed Dec. 21, 1951 Aug. 23, 1955 INVENTORS E' sssuvus DIEMER PIETER VAN BREE BY wW AGENT United States Patent Ofi Fice 2,716,199 Patented Aug. 23, 1955 ELECTRIC DISCHARGE TUBE FOR SHORT WAVES Gesinus Diemer and Pieter van Bree, Eindhoven, Netherlands, assignors to Hartford NationalBank and Trust Company, Hartford, Conn., as trustee Application December 21, 1951, Serial No. 262,706 Claims priority, application Netherlands January 26, 1951 2 Claims. (Cl. 313250) below 3 m., particularly of about 10 cms. and downwards. 1

Such tubes, which frequently comprise a disc-shaped cathode, exhibit the disadvantage that small variations of the operating temperature of the cathode, for example due to fluctuations of the mains voltage and hence of the cathode heating voltage, are liable to bring about inadmissible changes in the distance of the cathode relatively to the next following electrode. Small variations in the thermal expansion of the electrodes and stay members often bring about even inadmissibly large variations of the small distance of the cathode relatively to the next following electrode. Attempts have been made in a number of ways to neutralise or to prevent such variations. The disadvantage of the known constructions frequently resides in that satisfactory compensation can only be obtained for comparatively small temperature fluctuations.

The object of the present invention is to materially reduce this disadvantage. According to the invention an electric discharge tube for very short waves comprising a disc-shaped cathode which by means of a spacer is kept correctly spaced apart from the next following electrode, the spacer being heated substantially only by conduction and the said next following electrode, substantially only by radiation from the cathode, is characterized in that the coeflicients of expansion and the dimensions of the said spacer and of the said electrode and the point at which they are secured together or bear against each other are so chosen that at the operating temperature of the cathode, the expansion of the parts heated by thermal conduction is substantially neutralised by the expansion of the parts heated by radiation such that the distance between the cathode and the next following electrode remains substantially constant. This ensures that this distance remains substantially constant throughout the large region of different cathode temperatures so that fluctuations in the operating temperature of the cathode are substantially prevented from acting on the separation between cathode and next following electrode.

In order that the invention may be more clearly understood and readily carried into effect, it will now be described more fully with reference to the accompanying drawing, in which Figs. 1 and 2 show, by way of example, embodiments of a tube according to the invention, and

Fig. 3 shows the variation of the separation between cathode and next following electrode as a function of the cathode temperature.

Referring to Fig. 1, 1 designates the metal wall of a tube in which an anode 3 is sealed by means of a glass ring 2. The other end of the tube is sealed by a glass bottom part 4. A disc-shaped cathode 5 is urged by laminated springs 6 against a spacer 7 by which the distance of the cathode 5 relatively to the end 9 of anode 3 is determined when cold. The spacer 7 is secured to an insulating body 8 which at one end is secured to the anode 3 by glazing and the other end of which has been surface-ground simultaneously with the end 9 of anode 3.

On the cathode 5 being heated the insulating body 8 will be heated by thermal conduction. Expansion of this body increases the separation between cathode and anode. The more the cathode temperature is increased, the more intensely will increase, proportionally to T the heat radiation by which part 9 of the anode 3 is heated. The expansion of part 9 brings about a reduction of the cathode-anode distance so that this distance is the resultant of the expansion of the body 8 being heated by thermal conduction and the expansion of the part 9 heated by radiation. Since at higher cathode temperatures the radiation increases to a much greater extent than the thermal conduction, the part 9 will be heated by now to a greater extent than part 8. By favourably choosing the heat expansion coefficients of the parts 8 and 9 the variation, shown in Fig. 3, may be obtained, the curve which denotes the cathode-anode distance having a broad maximum and hence a substantially flat variation throughout a large region of Tk. If the conditions are chosen to be such that the maximum of this curve is coincident with the operating temperature of the cathode so that the normal operating temperature Tk of the cathode is located at the point W, even comparatively marked cathode temperature variations in the region between a and b will leave the cathode-anode distance substantially unchanged. The body 8 may be made from ceramic material having a suitable coetficient of expansion, for example by the ceramic material known under the registered trademark Kerpora, Whereas the anode is made for example of ferrochrome. Kerpora is a ceramic material consisting of about 91% by weight of steatite or soapstone 3MgO.4SiO2, about 3% of BaCOa, and between about 5 to 6% of SiOz, in addition to some flux materials and impurities.

An alternative embodiment is shown in Fig. 2, in which the surface of the cathode 10 is separated by the spacer plate 12 from the ceramic body 11, the lower surface of which is located in the same plane as the grid wires 13. These wires are stretched on a cylinder 14 which is gripped by the ceramic body 11 by means of a spring 16 in a slot of a sealing-in disc 15. Also in this case, the variations in separation between cathode and grid, which are due to the expansion of the ceramic body 11 are neutralised by the influence which the expansion of cylinder 14 exercises on this spacing. In this case, the said insulating body 11 may be made of hard glass and the metal cylinder 14 for example of molybdenum. The anode 17 extends within this cylinder 14 as far as the desired spacing from grid 13.

Apart from the said combinations of materials and embodiments, alternative combinations of materials may be chosen which with constructions altered to conform therewith permit of achieving the object of the invention. Structurally, the invention is particularly suitable for use with a diode having an indirectly heated disc-shaped cathode, in which a spacing between cathode and anode of 20 microns is possible.

What We claim is:

1. An electric discharge tube for very short waves comprising a cathode adapted to be displaceable in a given direction and having a planar electron emissive portion, means for heating the cathode to a given operating temperature, an electrode disposed opposite to and aligned with said emissive portion of said cathode and in thermalradiation relationship therewith, said electrode having dimensions and being constituted by a ferrochromium which a given change in dimension of the electrode inthe direction of said cathode is produced by changes in the cathode operating temperature, and a spacer member secured to a portion of-said electrode remote from said emissive portion, of said cathode and defining a given spacing between said cathode and said electrode, said spacer member being in thermal-conduction relationship with said cathode and having dimensions and being constituted by a ceramic material containing about 91% 3MgO.4SiOz, about 3% BaCO3, about 5 to 6% SiOz, and the remainder being flux materials and impurities and having a thermal coefficient of expansion at which a change in dimension of said spacer in the direction of said cathode is produced by the changes in the cathode operating temperature, the coefiicients of expansion of the spacer and the electrode having values at which the given direction and having a planar electron emissive portion, means for heating the cathode to a given operating temperature, an anode disposed opposite to and aligned with said emissive portion of said cathode and in thermal-radiation relationship therewith, said anode hav- :ill

4 ing dimensions and being constituted by a material having a thermal coeflicient of expansion at which a given change in dimension of the electrode in the direction of said cathode is produced by changes in the cathode operating temperature, and a spacer member secured to a portion of said anode remote from said emissive portion of said cathode and defining a given spacing smaller than 25 microns between said cathode and said anode, said spacer member being in thermal-conduction rela: tionship with said cathode and having dimensions and being constituted by material having a thermal coeflicient of expansion at which a change in dimension of said spacer in the direction of said cathode is produced by the changes in the cathode operating temperature, the coefiicients of expansion of the spacer and the anode having values at which the spacing between the cathode and the anode as a function of the cathode temperature is a maximum at said given operating temperature of the cathode, whereby the spacing between the cathode and the anode is maintained substantially constant;

References Cited in the file of this patent UNITEDSTATES PATENTS 2,146,365 Batchelor Feb. 7, 1939 2,190,668 Llewellyn Feb. 20, 1940 2,244,358 Ewald June 3, 1941 2,378,569 Messner et a1 June 19, 1945 2,462,921 Taylor Mar. 1,: 1949 

